A composition comprising magnetic metal oxide ultrafine particles, for example, so-called magnetic fluid, has various uses, and there can be mentioned, as one field thereof, a use as a base for diagnostic drugs.
However, in order to safely and efficaciously administer to living organisms metal oxide fine particles of macromolecular size, various factors must be considered. Known preparations have various drawbacks, particularly problems are recognized in the point of biocompatibility, and various proposals are made for improvement. For example, Japanese Tokuhyosho 500196/1989 (PCT/WO88/00060) discloses dextran-coated superparamagnetic fluid dispersed in a polycarboxylic acid buffer. For use of these magnetic fluid preparations in the medical field, some points including toxicity should still be improved.
In the application of magnetic fluid to medicine and diagnostic drugs, particularly as a result of research on the aspect of its toxicity, the present inventors found that magnetic fluid as a foreign substance within living bodies has a bad influence on living bodies, for example, it aggregates platelets which are an important constituent of blood, and this is a cause of toxicity of magnetic fluid.
Thus, the present inventors intensely studied aiming to develop a magnetic fluid which is free of side effects such as platelet aggregation, is excellent in safety to living bodies, and, when intravascularly administered, does not have a bad influence on living bodies. As a result, we have found that when an organic monocarboxylic acid, e.g. lactic acid is compounded into an aqueous sol of a complex of magnetic metal oxide ultrafine particles with a polysaccharide, a polysaccharide derivative and/or a protein, the property of the aqueous sol to aggregate platelets can remarkably be reduced without substantial change in the intrinsic properties of the aqueous sol such as magnetic properties, metabolic properties and tissue specificity, and completed this invention.